A method for cooling an electronic component by conducting heat, which is generated by driving the electronic component, to a heat sink is a conventionally used method. As a conventional structure of the heat sink, a technique for attaching a flat pipe (a cooling pipeline), through which a cooling medium flows, to a cooling plate has been disclosed. According to the technique, such a structure is adopted that the flat pipe is joined to the cooling plate via a long flat side portion of the flat pipe, and the heat that has transferred to the cooling plate is transferred to the flat pipe via a cross-sectional long side portion having a large heat transfer contact area (for example, see PTL 1).